Process for isomerizing naphthas



PROCESS FOR ISOMERIZING NAPHTHAS Filed Ivlay 12, 1959 DISTiLLATlON ZONEFLASH TOWER L- 32 PRODUCT DISTILLATION 4 REACTOR ZONE 7 4B a STAGE a na20 F" vls -1STAGE 2 IO n FEED STAGE M l L.! J

BY WAW Z INVENTORS PATENT ATTORNEY PROCESS FOR ISOMERIZING NAPHTHAS HughErwin Stanley, Peter Haynes Watkins, Richard Franklin Stringer, andMichael Francis McDonald, Sr., all of Baton Rouge, La., assignors toEsso Research and Engineering Company, a corporation of Delaware FiledMay 12, 1959, Ser. No. 812,613

Claims. (Cl. 260--683.67)

This invention concerns improvements in the catalytic isomerization ofparaffin hydrocarbons. More particularly, the invention relates toimprovements in the liquid phase conversion of normal or slightlybranched chain hydrocarbons of from 5 to 7 carbon atoms to commerciallyvaluable, more highly branched isomers, employing aluminum bromide asthe catalyst.

It is well known that the more highly branched isomers of the parafiinichydrocarbons occurring in petroleum gasoline fractions are more valuablethan the corresponding slightly branched or straight chain hydrocarbonsbecause of their higher octane ratings. The demand for motor fuels ofgreater octane number has increased markedly as the automotive industryhas provided gasoline engines with increasingly higher compressionratios to attain greater efficiency. One of the economically importantways in which the increased demands for higher octane fuels can be metis through the isomerization of the light naphtha components of suchfuels.

It may be generally stated that the isoparaffinic and branched chainparatfin hydrocarbons are of greater commercial value to the petroleumindustry than the corresponding straight chain hydrocarbons. Thus, forexample, 2,2-dimethyl butane has a higher octane rating than theisomeric normal hexane.

The isomerization of normal paraffin hydrocarbons of from 5 to 7 carbonatoms into the corresponding branched chain homologs is well known. Foreffecting the isomerization, it is customary to employ certain metalhalides, particularly aluminum chloride or aluminum bromide, inconjunction with certain promotors, such as hydrogen chloride, hydrogenbromide or boron fluoride. Insofar as the isomerization of lightnaphthas is concerned, the lower the temperature of isomerization,within limits, the more favorable is the equilibrium for convertingstraight chain paraffin hydrocarbons into isomers of high octane rating.Aluminum bromide has been found to be more active than aluminum chlorideat lower isomerization temperatures, e.g. in the range of about 50 toabout 120 F. The isomerization catalyst employed may be AlBr supportedon suitable carriers such as Porocel, bauxite, alumina, molybdena,molybdenaalumina, silica gel or the like. Alternatively, co-catalystsconsisting of complexes of aluminum bromide with ketones, alcohols,ethers, amines, inorganic acids, water or halogenated hydrocarbons maybe used. The cocatalysts, which are normally liquid at reactortemperature and are largely insoluble in hydrocarbon, may be employed instirred reactors or in pool reactors wherein the hydrocarbon feed ispassed by gravity flow upward through a pool of co-catalyst. Both ofthese catalyst systems have been found to convert naphtha to nearthermodynamic equilibrium distribution ofisomers in once-throughoperation. I

An important problem arising with the use of these highly activecatalysts is that they promote side reactions, such as cracking anddisproportionation. These side reactions are particularly evident athigh conversion condi- States Patent 0 ice tions. Extensive cracking anddisproportionation must be avoided since they cause severe catalystdeactivation. The side reaction problem is particularly serious inisomerization of heptanes.

It has in the past been proposed to minimize cracking by adding to thehydrocarbon feed to be isomerized, certain naphthenic hydrocarbons, suchas cyclohexane, methyl cyclopentanes, methyl cyclohexanes, and the like.Similarly, isobutane has been added to minimize cracking. Theseinhibitors, though helpful, are not completely successful in suppressingcracking to the low level necessary to maintain satisfactory catalystlife. Again, this is particularly evident in the case of the highermolecular weight hydrocarbon such as heptanes.

An important problem isomerizing naphthas, and in particular, virginnaphthas, to high octane motor fuel is that the components, inparticular the pentanes, hexanes, and heptanes, isomerize at differentrates. Thus heptane isomerizes at a substantially faster rate thanhexane, and the latter has a higher reaction rate than pentane. It hasbeen found that in the isomerization of a C -C naphtha in the presenceof an AlBl'g-HBI' support catalyst system and naphthalene andnaphthalene/isobutane cracking suppressors, the reaction is selective toisomerization rather than to cracking or sludging, up to 90 to 92%conversion of hexane to isomers. Further severity results in crackingand catalyst deactivation. It has further been determined that it isoptimum to convert pentane to isopentane, and C to isoheptane to producethe highest octane number product without cracking and catalystdeactivation. This is an extremely dillicult problem, because, under thesame reaction conditions, the C isomerization rate is twice that of Cisomerization and about 3.4 times that of the pentane isomerizationrate. Thus, when a virgin C -C naphtha is isomerized, the heptanereaches equilibrium conversion long before the hexane does, and thelatter substantially before the pentane reaches equilibrium. Forinstance, when C conversion to isoheptane is 90%, C conversion is only80% and C conversion is only 60%.

It would be possible to obtain high conversion of the individualhydrocarbon by operating the process under conditions to obtain highconversion of heptane. This would be followed by separating unconvertednormal pentane and hexane and recycling these to the reaction zone,either separately or together. Such a system, however, would beprohibitively expensive, for it would require one or more highlyefficient, expensive superfractionation towers to separate isopentanefrom normal pentane, and isohexane from normal hexane, and hexanes andpentanes from the heptanes and from one another.

It is the principal object of the present invention to provide a meansfor effectively suppressing extensive cracking and disproportionation ofhydrocarbons during isomerization in a high conversion Friedel-Craftsisomerization process so that the catalyst does: not become deactivatedin a manner such that high isomerization rates are effectivelymaintained.

It is a still further object of the present invention to provideconditions for isomerization so as to obta'n maximum yields of the mostdesirable isomers at maximum rates while minimizing degradation of theisomerization product and catalyst.

Other and further objects and advantages of the present tion. .If anunder cut C -C fraction is to beprocessed,

it is fractionated into individual streams comprising.

essentially pentane, hexane, and heptane. These streams are then fedseparately to a staged reactor or a series of reactors operated underconditions such that the pentane has .7 the longest residence time andthe heptane the shortest. Thus in one embodiment of the presentinvention, the pentane fraction is fed to a first reaction stage andreaction conditions controlled to isomerize to a yield of 50 to 60%isopentanes. This conversion is readily accomplished withoutencountering cracking or catalyst deactivation, even in the absence ofnaphthenic or. isobutane cracking inhibitors. The total pentane productfrom this stage is passed to a second stage to which is fed the Cfraction from the distillation tower. This feed normally containsnaphthene inhibitors occurring in the virgin naphtha, and isomerizationis continued in this stage to ac conversion of 75 to 85% isohexane, andthe pentane fraction added in the first stage is converted to 60 to 70%.This is controlled by careful adjustment of reactor conditions. Finally,the heptane fraction is added to the third stage wherein, during a verybrief residence time, equilibrium conversion is obtained withoutcracking or sludging. As will be seen more clearly hereinafter, eachstage is carefully controlled'as to reactor conditions, feed and productrates, and activity to obtain the desired conversion. As the feedconcentrations vary in C C and C hydrocarbons, these reactor stages canbe varied to obtain equilibrium conversion on each fraction;temperature, promoter concentration, and stage size may be varied toobtain the desired goal. The reactor stages may be separate vessels orbe segregated portions of a single reactor.

The-nature and objects of the present invention will be more readilyunderstood when reference is made to the schematic drawing depicting aflow plan of a suitable process embodying the principles of the presentinvention. The description is particularly directed to the use ofaluminum bromide as the preferred catalyst, although the process is notto be limited thereto.

Referring now to the drawing, a feed stream comprising.C C parafi'inichydrocarbons, preferably from which aromatics have been removed, is sentthrough line 10 into a splitter tower 12. Overhead a C fraction iswithdrawn through line 14, a C fraction is withdrawn through line 16,and a C fraction throughline 18, respectively.

The pentane fraction is now passed to stage 1 of isomerization reactorand a relatively low conversion,

from 60 to- 70% of the pentane feed, is obtained in this.

stage.

The reaction zone throughout the three stages may contain a bed ofsuitable support material for the aluminum bromide, such as alumina,silica, bauxite or the like, or it may contain a liquid catalystconsisting of a complex of aluminum bromide with an alcohol, ketone,amine, water, inorganic acids, halogenated hydrocarbons, orhydrocarbons. With the liquid aluminum halide complex catalyst somemethod such as an agitator must be provided for intimately contactingthe liquid catalyst with the hydrocarbon feed. When employing a solidsupport such as alumina or bauxite for the aluminum bromide, the supportwill adsorb 10 to 80% aluminum bromide based on support weight. Thesupported aluminum bromide catalyst is formed by introducing aluminumbromide at l to wt. percent concentration in the initial portion of thefeed. The aluminum bromide complex catalyst may be formed externally andadded to the reaction zone or it may be formed by adding thealuminumbromide at 1 to 30- wt. percent concentration in the. initialportion of the feed. After formation of the supported or complexedaluminum bromidecatalyst, the amount of aluminum bromide added in thefeed may then be reduced to the quantity of AlBr that will be con vertedto sludge. in the process by undesirable'side're-' actions.The:aluminumbromidemake-upzm :be addedslm by dissolving the AlBr in aportion of the hydrocarbon feed to be introduced through line 14.

Also introduced with the feed in an amount of hydrogen halide promotersufiicient to maintain a positive pressure of hydrogen halide of from 1to 30 wt. percent in the reaction zone. Initially,this promoter isintroduced through line 22 and after the process is under way, the majorportion of the halide will be introduced through recycle line 34 whilemake-up promoter-will enter through line" 22.

if desired, isobutane is added to the feed in reactor 20 by recoveringthe isobutane from the product and recycling it through line 34 to thereaction zone. Furthermore, cracking suppressor naphthenes found invirgin naphthas may be introduced with the feed or separately throughline 22. It is desired to maintain in reaction stage 2 from 2 to 50 vol.percent, based on feed to be isomerized, of naphthenes and from 25 to100 vol. percent of isobutane. However, as pointed out, it is generallymore desirable to add these cracking suppressors at later stages, sincethe pentane may be isomerized to the low conversion level of 60 to 70%without significant cracking.

The feed is conducted through stage 1 of reaction zone 20 at rates ofthe order of 0.1 to 4.0 v./v./hr. Reactor conditions includetemperatures of 50 to 400 F., preferably 100 to 200 F.

The desired degree of conversion of C in stage 1 iscontrolled by varyingone or more of the following van ables; v./v./hr., reactor temperature,and/ or AlBr or HBr in feed solution.

As pointed out previously, side reactions such as cracking anddisproportionation are low at low conversionswhen the product containsabout 92% isohexane based Accordingly, along with on the totalparaflinic hexanes. the n-hexane fraction introduced into stage 2 of thereactor 20 there is added a cracking suppressor of the kind previouslyenumerated. methylcyclohexane or cyclohexane, may be added to the extentof 2 to 50%. preferably 10 to 20%, of the hexane; while isobutane may besimilarly added to the extent of 10 to 500%, preferably 25 to 100%.These suppressors may be added to the feed along with the C fraction asabove, or be introduced through line 22.

The partially converted pentane fraction is now passed into stage 2along with the fresh hexane, as well as the catalyst and promoter. Thereaction conditions in stage 2 are :similar to those in stage 1.However, they may differ depending on the relative concentrations of Cand C paraffins in the feedstock being processed and the conversionlevel of these paraffins in said feedstock.

In the final stage 3 there is passed the fresh 0-; fraction from line 18as well as the reaction products from'stages 1 and 2.. Here the reactionis carried out such that substantially equilibrium products are obtainedof each fraction.

of these paraffins in said feedstock.

The total isomerization product, which will contain some dissolvedaluminum bromide, is removed from the reactoi and conducted to a flashtower 24, wherein 50 to 95% of the product will be'flashedoverhead'through line'25. This product will be hydrogen halide,isobutaneand naphtha product. The bottomsfrom the flash zone, whichcontain-dissolved aluminum bromide and hydrocarbons, are recycledthrough line 13 to the feed stream entering reactor 20 and thus willeffect return of -the cam-- lysta to theaeaetoni Cdnditionsin the'flashzoneiarea- A naphtheue, such as 50 to 200 mm. Hg vacuum to 500p.s.i.g. and 100 to 400 F.

The product flashed overhead from tower 24 is conducted to adistillation tower 30, wherein the hydrogen halide promoter, togetherwith the isobutane is distilled from the product and recycled throughline 34 into the feed stream entering the reactor 20. Tower 30 may beoperated at unit pressure or it may if desired be operated at lowerpressures.

The hydrogen halide activators, of which hydrogen bromide is preferred,are necessary not only to promote the isomerization but also to preventdeactivation of the catalyst.

The process of the present invention may be subject to many variationswithout departing from its spirit. Thus, under certain conditions, itmay be preferable to fractionate the virgin naphtha and, instead ofisomerizing the 0, fraction, send the latter to a catalytic hydroformingzone wherein it is converted to aromatic in the presence of a platinumcatalyst and hydrogen. In such case only a two-stage isomerizationprocess is necessary, the first stage converting the pentane to 60 to75% isopentanes, and the second converting the partially conver-tedpentanes as well as the n-hexane stream to equilibrium product of 92%i-C and 80% i-C without cracking. Furthermore, as an alternate, thethird stage may be two separate vessels, the first operating at atemperature of 200 to 400 F. to complete the conversion to equilibrium,and the second at a temperature of 50 to 150 F. to obtain a morefavorable product distribution. Equilibrium at lower temperatures ismore favorable to higher conversion to isomers.

The following specific example illustrates the benefits to be derivedfrom the process of the present invention.

Example The advantages of operating in accordance with the describedprocess were demonstrated in two laboratory bench scale experiments. Inthe first experiment, a C -C fraction from a South Louisiana crude wasreacted over AlBr -Porocel catalyst in the presence of HBr. In thesecond experiment, the C /C cut of the C -C fraction used in Experiment1 was isomerized over AlBr Porocel catalyst in the presence of HBr from62% C isomers on C paraffins to 85% C isomers. The partially converted C/C cut was then blended with the C from the original C C fraction. Thismaterial was then passed to a second reaction stage where the Cconversion was increased from 85 to 92%. The second experiment, which 18in accordance with the described process, gave only 1% cracking whileExperiment 1 gave 4% cracking. Data for these experiments are shown inthe table below:

Experi- Experi- Eliotlt trout 1 2 Feed:

C5 Conversion to Tsomers, Vol. percent 30 36 Ct Conversion to Isomers,Vol. percent- 62 62 C7 Conversion to isomers, Vol. percent- 65 66Product of Reaction Stage 1:

C Conversion to Isoulers, Vol. percent 49 C Conversion to ISOLT ers,Vol. percent 85 Product. of Reaction Stage 2:

0.; Conversion to Isorrers, Vol. percent 70 Ca Conversion to Isoners,Vol. 92 92 C Con ersion to Isoniers, Vol. 96 92 Cracking to 0 minus.Vol. percent 4 1 A practlcal commercial isomerizatio-n process willtolerate up to about 1% cracking to G, minus. However,

the 4% cracking shown in Experiment 1 will produce a very rapid catalystdeactivation rate which is not tolerable in a commercial process.

What is claimed is:

1. An improved process for isomerizing a light naphtha containingparaflinic hydrocarbons having from 5 to 7 carbon atoms into more highlybranched isomers which comprises: separating a C /C fraction and a Cfraction from said naphtha, isomerizing said first named fraction withan aluminum bromide comprising catalyst in a first liquid phaseisomerization zone, controlling isomerization conditions of residencetime, temperature, and feed rate to produce a product containing notmore than isohexane, based on C parafiins, withdrawing said partiallyconverted fraction and catalyst from said first stage and passing saidmixture to a second stage isomerization zone, passing a naphtheniccracking suppressor and at least a portion of said O; fraction to saidzone, continuing said isomerization process to produce an equilibriumreaction product from both fractions, and recovering a high octaneproduct.

2. The process of claim 1 wherein said isomerization temperature ismaintained in the range of from about 50 to about 150 F. and said feedrate at about 0.1 to 4.0 v./v./hr.

3. The process of claim 1 wherein about 2 to 50% of naphthenichydrocarbons, based on hexane feed, is passed into said first and secondzones.

4. The process of claim 1 wherein 25 to 100% isobutane, based on hexane,is passed into said first zone.

5. The process of claim 1 wherein said catalyst is AlBr; supported on asolid oxide carrier, and gaseous HBr is employed as a catalystactivator.

References Cited in the file of this patelit UNITED STATES PATENTS2.386.784 Fragen Oct. 16, 1945 2,443,607 Evering June 22, 1948 2,530,875Gwynn et al. Nov. 21, 1950

1. AN IMPROVED PROCESS FOR ISOMERIZING A LIGHT NAPHTHA CONTAININGPARAFFINIC HYDROCARBONS HAVING FROM 5 TO 7 CARBON ATOMS INTO MORE HIGHLYBRANCHED ISOMERS WHICH COMPRISES: SEPARATING A C5/C5 FRACTION AND A C7FRACTION FROM SAID NAPHTHA, ISOMERIZING SAID FIRST NAMED FRACTION WITHAN ALUMINUM BROMIDE COMPRISING CATALYST IN A FIRST LIQUID PHASEISOMERIZATION ZONE, CONTROLLING ISOMERIZATION CONDITIONS OF RESIDENCETIME, TEMPERATURE, AND FEED RATE TO PRODUCE A PRODUCT CONTAINING NOTMORE THAN 85% ISOHEXANE, BASED ON C6 PARAFFINS, WITHDRAWING SAIDPARTIALLY CONVERTED FRACTION AND CATALYST FROM SAID FIRST STAGE ANDPASSING A NAPHTHENIC CRACKING STAGE ISOMERIZATION ZONE, PASSING ANAPHTHENIC CRACKING SUPPRESSOR AND AT LEAST A PORTION OF SAID C7FRACTION TO SAID ZONE, CONTINUING SAID ISOMERIZATION PROCESS TO PRODUCEAN EQUILIBRIUM REACTION PRODUCT FROM BOTH FRACTIONS, AND RECOVERING AHIGH OCTANE PRODUCT.